arth
were to cease.

"There is another way to illustrate it," he continued. "Suppose there
was a large mass of lead, as big as a load of hay, hanging by a chain;
and also a great puff of feathers, or a balloon of the same size,
hanging in the same way. Now, if they were both suspended freely, they
would both move easily, for their weight would be supported by the
chain; but the heavy one would move very slowly. Nathan could move it,
but he could only move it slowly and a little way."

"I should not think that he could move it but very little," said Rollo.

"No, he could not; because you see that, in that way of suspending
anything, the moment that it begins to move, it begins to swing off and
to rise; so that it cannot be moved at all without being _lifted_ a
little. And the more it is moved, the higher it is lifted, so that it
would take a great force to move it far away from the centre, where it
was hanging. But we can hang it in a way to avoid that difficulty."

"How, sir?" said Rollo.

Rollo seemed to be very much interested in this conversation. He had
dismounted from his father's knee, and stood by his side, listening
eagerly. His mother, too, was paying close attention. As for Nathan, he
sat still; though it is not by any means certain that he understood it
very well.

"Let us suppose," said his father, "that the mass of lead, as big as a
load of hay, is fastened to one end of a stick of timber."

"That would not be strong enough to hold it," said Rollo.

"Well, then, to a beam of iron, as large as a stick of timber," rejoined
his father.

"O," said James, "you could not get such a big bar of iron."

"No," replied his father, "only an imaginary one; and that will be just
as good as any. Now, suppose the great mass of lead is fastened to one
end of this bar, and another one, just like it, to the other end, to
balance it. Now, suppose that the lower end of the great chain is
secured around the middle of the iron beam, and the upper end to be
fastened to some strong support up in the air. Now, we can move the mass
of lead without having to lift it at all; for, if we push against it,
and make it move, it will move round and round, without rising at all,
as it did before, when it was hung up directly by the chain."

Rollo's father then went on to explain to them that, in such a case as
this, the weight of the two masses of lead would not prevent their
moving easily, for they would exactly balance each oth